Portable motorized floor sanders for resurfacing wood floors and the like have been known for many years. These sanders typically include a frame, two or more wheels on the frame, sometimes including a caster wheel, a sanding head which is carried by a chassis mounted on the frame, and a motor for rotating the sanding head to sand the floor. In some sanders the chassis is pivoted on the frame so that the sanding head can be pivoted up and down relative to the surface to be sanded.
Generally, there are two types of portable floor sanders, drum sanders and continuous belt sanders. In a drum sander an abrasive such as sandpaper is secured to the drum which is rotated by a motor. As the drum rotates, the sandpaper rotatably contacts the surface of the floor to effect the sanding operation. In a continuous belt sander, a continuous loop or "belt" of an abrasive such as sandpaper is trained around a drum, known as the contact wheel, and a tracking roller. A tracking and release mechanism maintains the abrasive belt aligned on the contact wheel. The contact wheel is typically driven by a motor and V-belt arrangement. The present invention is applicable to both types of sanders, that is, to drum sanders and to continuous belt sanders. To obtain a smooth, level finish, it is important that the sanding head (whether it be the drum of a drum sander or the contact wheel of a continuous belt sander) remain "level" with respect to the wheels of the sander. In other words, it is important that the axis of rotation of the sanding head be coplanar with the axis of the wheels of the sander. If these axes are not maintained coplanar (i.e., if the sanding head is skewed vertically with respect to the axis of the wheels), forces are generated which tend to cause non-uniform sanding along the head of the sander. Moreover, such forces tend to produce a sideward drag on the sander, causing it to pull sideways rather then track straight ahead, and further causing uneven wheel wear. In a continuous belt sander, these forces also place undue strain on the tracking device holding the belt aligned on the head. There are some situations when it is desirable that the axes of rotation of the sanding wheel and the truck wheels not be parallel. However, this is the exception rather than the rule.
In the prior art, various schemes have been developed to maintain the sanding head "level" with respect to the wheels of the sander. For example, in one prior design a pivot shaft is mounted on the frame of the sander, and a chassis carrying the sanding head is mounted on the pivot shaft for pivotal movement relative to the frame to raise and lower the sanding head. An eccentric on one end of the pivot shaft is used to cant the chassis until the axis of the sanding head is coplanar with the axis of the wheels.
In another prior design, the sanding head is mounted for rotation on the chassis by two suspension arms. Leveling of the head is accomplished by raising one arm relative to the other arm.
However, these and other prior leveling mechanisms suffer substantial drawbacks. Some prior designs cause skewing of the sanding head relative to the direction of movement of the sander, which causes the sander to track sideways instead of straight ahead as it moves forward. Some cause uneven loading of the sanding head along the length of the head, which results in an uneven surface finish. Some cause misalignment of the drive components and diminish rigidity of the sanding head, which causes periodic vibration and chatter of the sander as it moves across the floor. In short, prior leveling mechanisms have diminished the quality of sanding performance and/or placed undue stress on components of the sander. This is especially true in the case of continuous belt sanders.
There is a need, therefore, for an improved leveling mechanism which can be used on portable sanders and which is operable to maintain the sanding head generally parallel or "level" with the axis of rotation of the wheels of the sander, without suffering the disadvantages associated with the prior designs.